The present invention relates to the control of the flow of refrigerant in a refrigeration or air conditioning system and particularly relates to control of refrigerant flow in mobile air conditioning systems of the type employed for cooling the passenger compartment in a vehicle. Heretofore, it has been common practice to employ a temperature responsive expansion valve for controlling flow of refrigerant to the endothermic heat exchanger or evaporator in the refrigeration or air conditioning system, which valve employed a fluid filled control chamber responsive to evaporator discharge temperature to create pressure on a diaphragm connected to operate a moveable valve member for controlling flow to the evaporator inlet. Such temperature responsive valves are commonly referred to as thermostatic expansion valves and have enjoyed widespread use in high volume production of passenger automobiles because of the relative low cost of manufacturing of such valves. However, such thermostatic expansion valves do require that the temperature sensing fluid chamber be hermetically sealed with the sensing fluid and thus have been prone to developing leakage in the sensing chamber over prolonged service.
Although temperature responsive thermostatic expansion valves have proven generally satisfactory for controlling refrigerant flow to the evaporator, such a device has the disadvantage that it can only react to the result of a change in ambient conditions or thermal load on the evaporator for control purposes; and, therefore control of the refrigerant flow lags the need for change. Thus, it has been desired to provide an electrically controlled expansion valve which can be controlled by a signal generating circuit which employs a "smart" control signal developed by a microcomputer receiving inputs from sensors identifying the changes in ambient conditions. Such an arrangement with an electrically controlled expansion valve can thus be designed to anticipate the need for change in refrigerant flow responsive to extreme or rapid changes in ambient conditions or thermal load on the evaporator. The problem of providing refrigerant flow control in a passenger vehicle air conditioning system in response to extreme or rapid change of ambient conditions or thermal load on the evaporator is particularly complicated or aggravated by the rapid changes in the speed of the compressor which is normally driven by a belt connected to the vehicle engine power shaft. Thus, in extreme conditions of sun load and high ambient temperatures with the vehicle engine idling, full flow is required; whereas, as the engine speed comes off idle sudden throttling is required in view of the almost instantaneous increase in refrigerant flow as compressor RPM increases with engine speed.
Thus it has been desired to provide a way or means or strategy for controlling an electrically operated expansion valve which is particularly suited to global air conditioning systems where rapid changes in ambient conditions are encountered.